the voltage is the same on all batterys, its the AH you need to worry about, and maybe the cold cranking amps...
For instance i think mine has a 45AH with something like 470 which is pretty good.. "aquired" that battery from a bmw mini
my bmw 325 i "aquired" a battery from a new car with stop/start so it has a wopping ... 90AH 900 cold cranking amps. and it only just fits
if your doing it for ICE then some of those flux capacitor thingies is probably best with a battery a little bit bigger than yours is currently ie if you got 45AH go for 55AH kinda idea, but i understand that it can get to the point where the alternator isnt good enough to keep a big fooking battery charged?